Jenny Harris, of Balclutha Primary School, Balclutha, Otago, asks :-
I left some strawberries in the garden. They went fluffy. Is this because the seeds are sprouting or because they are going mouldy?
Monika Walter, a berry pathologist at Hortresearch, Lincoln, responded.
The 'fluffyness' observed on the strawberries was mould, most likely caused by the fungus Botrytis cinerea also known as grey mould (if the 'fluff' was greyish and 'velvet-like'). If the 'fluff' was more like 'cotton wool', then the mould could have been caused by a fungus from the Rhizopus genera. But Botrytis cinerea is more common and it is one of the most destructive diseases on strawberries worldwide.
Botrytis cinerea mycelium readily grows on the dead leaves of the strawberry plant and on surrounding weeds. This mycelium is usually the main source of disease inoculum. Under humid conditions - like after rain or dew - the Botrytis mycelium forms conidiaphores (a black 'stalk') bearing spores. This is termed sporulation. Each conidiaphore with its spores looks a bit like a miniture tree: central trunk, with branches on top and the spores at the tip of the branches, spores bunched in clusters. To see the conidiaphores a good magnifiying glass will help - to see the spores a lower powered microscope is essential.
These `miniture trees' (condiaphores and spores) aranged in a `forest' makes the infected fruit look 'fluffy'. The spores then can be picked up by wind (ie become airborne) or insects and find new flowers, dead leaves, or fruit to infect. If conditions are right, the cycle from infection to sporulation can happen within a week.
There is no easy way to control Botrytis cinerea. Control involves inoculum mangement (eg removal of dead leaves, weeds and all infected berries/tissues) as well as the use of fungicides/biological control agents to suppress mycelial growth, sporulation or to kill the spores.
Fruit management is also important. It usually involves avoiding injury, harvesting at the right 'ripeness' and appropriate storage (near to 0 degrees Celsius) as this will slow the growth of the fungus.
Strawberry seeds do germinate. They, however, need some priming hence strawberries are rarely propagated from seeds. Only plant breeders grow plants from seeds to develop new varieties. Strawberry plants are usually propagated by their runners or in tissue culture.